Silver-halide photographic elements contain light sensitive silver halide in a hydrophilic emulsion. An image is formed in the element by exposing the silver halide to light, or to other actinic radiation, and developing the exposed silver halide to reduce it to elemental silver.
In color photographic elements, a dye image is formed as a consequence of silver halide development by one of several different processes. The most common is to allow a by-product of silver-halide development, oxidized silver-halide developing agent, to react with a compound called a coupler to form the dye image. The silver and unreacted silver halide are then removed from the photographic element, leaving a dye image.
In either case, formation of the image commonly involves liquid processing with aqueous solutions that must penetrate the surface of the element to come into contact with silver halide and coupler. Thus, gelatin and similar natural or synthetic hydrophilic polymers have proven to be the binders of choice for silver-halide photographic elements. Unfortunately, when gelatin or similar polymers are formulated so as to facilitate contact between the silver halide crystal and aqueous processing solutions, the resultant coatings are not as fingerprint and stain resistant as would be desired for something that is handled in the way that an imaged photographic element may be handled by various persons various times and in various circumstances. Thus, fingerprints can permanently mark the imaged element. The imaged element can be easily stained by common household products, such as foods or beverages, for example, coffee spills.
There have been attempts over the years to provide protective layers for gelatin-based photographic systems that will protect the images from damage by water or aqueous solutions. U.S. Pat. No. 2,173,480 describes a method of applying a colloidal suspension to moist film as the last step of photographic processing before drying. A number of patents describes methods of solvent coating a protective layer on the image after photographic processing is completed and are described, for example, in U.S. Pat. Nos. 2,259,009, 2,331,746, 2,798,004, 3,113,867, 3,190,197, 3,415,670 and 3,733,293. More recently, U.S. Pat. No. 5,376,434 describes a protective layer formed on a photographic print by coating and drying a latex on a gelatin-containing layer bearing an image. The latex is a resin having a glass transition temperature of from 30.degree. C. to 70.degree. C. Another type of protective coating involves the application of UV-polymerizable monomers and oligomers on a processed image followed by radiation exposure to form crosslinked protective layer, which is described in U.S. Pat. Nos. 4,092,173, 4,171,979, 4,333,998 and 4,426,431. A drawback for both the solvent coating method and for the radiation cure method is the health and environmental concern of those chemicals or radiation to the coating operator. Another drawback is that the photographic materials need to be coated after the processing step. Thus, the processing equipment needs to be modified and the personnel running the processing operation need to be trained to apply the protective coating.
Various lamination techniques are known and practiced in the trade. U.S. Pat. Nos. 3,397,980, 3,697,277 and 4,999,266 describe methods of laminating a polymeric sheet film, as a protective layer, on a processed image. U.S. Pat. No. 5,447,832 describes the use of a protective layer containing a mixture of high and low Tg latices as a water-resistant layer to preserve the antistat property of a V.sub.2 O.sub.5 layer through photographic processing. This protective layer is not applicable to the image formation layers, however, since it will detrimentally inhibit the photographic processing. U.S. Pat. No. 3,443,946 provides a roughened (matte) scratch-protective layer, but not one designed to be water-impermeable or resistant. U.S. Pat. No. 3,502,501 is intended to provide protection against mechanical damage only; the layer in question contains a majority of hydrophilic polymeric materials, and must be permeable to water in order to maintain processability. U.S. Pat. No. 5,179,147 likewise provides a layer that is not water-protective.
Protective coatings that need to be applied to the image after it is formed, several of which were mentioned above, adds a significant cost to the final imaged product. A number of patents have been directed to water-resistant protective coatings that can be applied to a photographic element prior to development. For example, U.S. Pat. No. 2,706,686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the light-sensitive layer, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or dissolution during processing. The overcoat as described is coated as a suspension in an organic solvent, and thus is not desirable for large-scale application. More recently, U.S. Pat. No. 5,853,926 to Bohan et al. discloses a protective coating for a photographic element, involving the application of an aqueous coating comprising polymer particles and a soft polymer latex binder. This coating allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. Again, however, the hydrophobic polymer particles must be fused to form a protective coating that is continuous and water-impermeable.
Protective coatings containing gelatin-grafted polyurethanes using pendant carboxylic acid groups on the polyurethanes for grafting, further in combination with polyvinyl alcohols, have been disclosed for use as overcoats on the non-emulsion side of photographic elements. See, for example, U.S. Pat. No. 5,910,401 and 5,846,699. U.S. Pat. No. 5,846,699 discloses a mixture of a polyurethane and a carboxylic acid containing polymer having an acid number of 60 to 260, to provide resistance to abrasion and the like, for example on the backing of a photographic element.
Polyurethane-containing interpenetrating networks have been used in coatings for paper. See, for example, U.S. Pat. No. 5,177,128. None of these patents, however, disclose interpenetrating polymer networks comprising polyurethane or polyurethanes with an acid number of at least 5 in a protective coating applied over a silver-halide emulsion layer of a photographic element.
The ability to provide the desired property of post-process water/stain resistance of an imaged photographic element, at the point of manufacture of the photographic element, is a highly desired feature. However, in order to accomplish this feature, the desired photographic element should be permeable to aqueous solutions during the processing step, but after processing achieve water resistance and even water impermeability for at least some time after contact with water. U.S. Ser. No. 09/235,436 disclosures the use of a processing solution permeable overcoat that is composed of a urethane-vinyl copolymer having acid functionalities. However, the limitation of coating such a polymer is that, at coverages desired for durability, the overcoat tends to exhibit defects such as cracks which are formed during the coating process. In addition, the presence of the overcoat causes a slight decrease in the permeation and reaction rates of the developer with the light sensitive emulsions in the underlying layers, resulting in a greater possibility of variability in image-qualily. U.S. Ser. No. 09/235,437 discloses the use of a second polymer such as a gelatin or polyvinyl alcohol to reduce such defects and disadvantages. U.S. Ser. No. 09/448,213 discloses further improvements by selection of the polyvinyl-alcohol component in the overcoat.
Therefore, there remains a need for, and it would be highly desirable to obtain, a protective overcoat for an photographic element that would not significantly reduce the rate of reaction of the developer with the underlying emulsions, but which would also provide a water resistant and durable overcoat after the processing or developing step. Furthermore, there is a need for a commercially viable water-resistant coating that can be applied to an photographic element prior to exposure and which is permeable to water during development and which becomes relatively impermeable to water in the final product without necessitating a fusing step.